Corals are indeed hard, but this hardness varies depending on the type of coral. Stony corals, which build reefs, have hard skeletons made of calcium carbonate. These skeletons form the rigid structure of coral reefs that we see. The hardness comes from the polyps secreting calcium carbonate to create their protective outer skeletons. Soft corals, on the other hand, are more flexible and don’t have the same hard structure as stony corals.
Corals grow in warm, shallow waters of tropical oceans around the world. They thrive in areas where sunlight can penetrate the water, typically at depths less than 150 feet (45 meters). You can find coral reefs in the Caribbean Sea, the Indian Ocean, the Pacific Ocean, and the Red Sea. The Great Barrier Reef off the coast of Australia is the largest coral reef system in the world. Corals need clear, clean water with temperatures between 68-82°F (20-28°C) to grow well. They also require a solid surface, like rocks or other hard substrates, to attach themselves and begin building their colonies.